Centralize blockchain transaction building into a single service

[martinjms]

Summary

Transaction-building logic is currently spread across multiple components (Overview, Buildings). Each independently resolves the binding ID, builds PTBs, and handles results. This led to a critical bug where the binding ID wasn't loaded correctly in one place, causing Apply to create duplicate bindings.

Proposed solution

Create a single TransactionService or useTransactions hook that:

• Owns the binding ID resolution (single source of truth)
• Validates binding ID exists before any transaction
• Provides typed methods: install(), updateUrl(), applyPolicies(), addToBlocklist(), removeFromBlocklist()
• Handles result parsing, toast notifications, and query invalidation in one place
• Prevents duplicate binding creation

Benefits

• Binding ID validated in one place, not scattered across components
• Consistent error handling and user feedback
• Easier to implement single-transaction Apply (Single-transaction Apply flow #5) later
• Reduces code duplication between Overview and Buildings pages

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